The present invention relates to the x-ray tube art. It finds particular application in conjunction with high power x-ray tubes for use with CT scanners and the like and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with conventional x-ray diagnostic systems and other penetrating radiation systems for medical and non-medical examinations.
Typically, a high power x-ray tube includes an evacuated envelope or housing which holds a cathode filament through which a heating or filament current is passed. A high potential, typically on the order of 100-200 kV, is applied between the cathode and an anode which is also located within the evacuated envelope. This potential causes a tube current or beam of electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. The electron beam impinges on a small area or focal spot of the anode with sufficient energy to generate x-rays.
In order to increase the resolution of a CT scanner, it is desirable to modulate the position or size of the focal spot between two or more positions or sizes, creating two distinct point sources of radiation. Conventionally, two different methods have been employed to control the position and/or width of the focal spot. One method of focal spot control employs electrostatic grids or biasing electrodes referenced to a common leg of a single filament. The voltages on the two electrostatic grids are varied to change the location, as well as the width, of the electron beam impinging on the focal track of the anode. While the electrostatic method yields greater focal position control, it is limited to providing a focal spot of a single length.
Another method of focal spot control employs a magnetic yoke in order to create a magnetic field that affects the path of the electron beam emitted from the cathode. While the magnetic yoke method employs two filaments, therefore providing two focal spot lengths and widths, it is disadvantageous for a number of reasons. The magnetic yoke tube requires two additional connections to be passed through the x-ray tube housing, making it incompatible with many CT systems. In addition, the magnetic fields employed to deflect and focus the electron beam cannot be moved in a square wave fashion between the two focal spot positions, creating a gap in the collected data.
Therefore, a need exists for an x-ray tube assembly that provides multiple focal spot lengths and widths to create a system having a high modulation transfer function as well as a high x-ray flux in order to limit exposure times. The present invention contemplates a new and improved x-ray tube having an adjustable focal spot length and width, which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, an x-ray tube assembly includes an evacuated envelope and an anode disposed at a first end of the evacuated envelope for rotation about an anode axis. A cathode assembly disposed at a second end of the evacuated envelope emits an electron beam which strikes the anode at a focal spot, having a focal spot length and a focal spot width. The cathode assembly includes a variable-length filament assembly which emits electron beams, which impinge on the anode at focal spots having variable lengths. A cathode cup defines a plurality of electrostatic deflection electrodes which are electrically insulated from each other. Further, potentials are individually and selectively applied to different ones of the electrostatic electrodes of the cathode cup for controlling the width and location of the focal spot on the anode.
In accordance with another aspect of the present invention, an x-ray tube includes a cathode assembly having a long filament portion and a short filament portion and a common electrostatic deflection electrode disposed between the long and short filament portions. A first electrostatic deflection electrode is disposed adjacent the long filament portion opposite the common electrode and a second electrostatic deflection electrode is disposed adjacent the short filament portion opposite the common electrode. The x-ray tube further includes an anode and a vacuum enclosure which encloses the cathode assembly and the anode.
In accordance with another aspect of the present invention, an x-ray tube with an adjustable length and width focal spot includes an anode and a cathode assembly, which includes at least two filament segments and electrostatic deflection electrodes. A vacuum envelope surrounds the cathode assembly and the anode. Not more than four leads pass through the vacuum envelope to apply electrical power to the filament sections and bias potentials to the electrodes. A filament selection circuit is disposed inside the vacuum envelope in connection with the four leads passing through the vacuum envelope. The filament selection circuit is connected with the filament segments for applying electric current selectively through a long section of filament and a short section of filament in order to control focal spot length. Further, the filament selection circuit is connected with the electrodes in order to select focal spot width end position.
In accordance with another aspect of the present invention, an x-ray tube assembly includes an evacuated envelope having an electron-emitting cathode assembly spaced apart from a rotating anode, where the cathode assembly includes at least a first filament and a second filament for emitting electrons in a beam which impinges on the anode at a focal spot having a variable length and a variable width. A cathode cup is sub-divided into at least three electrically insulated deflection electrodes. A filament select circuit is disposed adjacent the evacuated envelope. The filament select circuit includes means for selectively and individually electrically heating one of the first and second filaments and means for individually and selectively applying potentials to different ones of the electrostatic deflection electrodes in order to control a width and a location of a focal spot on the anode.
In accordance with another aspect of the present invention, a computerized tomographic system includes a source of penetrating radiation for transmitting radiation through a subject disposed in a subject receiving aperture. The source includes at least two point sources of radiation, each providing beams of radiation having different focal lengths. Detector means are coupled to the source for detecting radiation emitted from the source after passage of the radiation through the subject. The source and detector means are mounted on a rotatable gantry. The system further includes means for processing the detected radiation into a tomographic image representation.
In accordance with a more limited aspect of the present invention, the source of penetrating radiation includes an evacuated envelope and an anode disposed at a first end of the evacuated envelope. A cathode assembly is disposed at a second end of the evacuated and includes a cathode base portion and at least a first filament and a second filament, where the first filament is longer than the second filament. At least three deflection electrodes are attached to and electrically insulated from the cathode base portion. The source further includes means for individually and selectively applying potentials to different ones of the deflection electrodes.
In accordance with another aspect of the present invention, an x-ray tube includes an evacuated envelope having a cathode spaced apart from an anode adapted to be maintained at a positive voltage relative to the cathode. The cathode includes a filament assembly for selectively emitting electrons in a beam which impinges on the anode at a focal spot having at least one of a long focal spot length and a short focal spot length and a variable focal spot width, and a cathode cup having a plurality of parts electrically insulated from each other. A method of operating the x-ray tube includes the steps of selectively heating a portion of the variable filament assembly to emit electrons in the beam having one of the short focal spot length and the long focal spot length. The method further includes individually and selectively applying potentials to different ones of the cathode cup parts for controlling the width and location of the focal spot on the anode.
One advantage of the present invention resides in obtaining a higher x-ray flux without overheating the anode track.
Another advantage of the present invention is that it produces x-ray radiation having multiple focal spot lengths.
Another advantage of the present invention resides in the presence of multiple filaments without additional external connections between the x-ray tube and the CT system.
Another advantage of the present invention resides in the combination of filament length selection and electrostatic focusing.
Yet another advantage of the present invention resides in selective excitation of one of multiple filaments.
Still another advantage of the present invention is that it modulates the focal spot between two or more positions providing greater sampling density.
Other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.